Correlation between oxygen consumption calculated using Fick's method and measured with indirect calorimetry in critically ill patients.

Objective: To compare the oxygen consumption index measured by using indirect calorimetry (VO2I Delta) with a portable metabolic cart and calculated according to Fick's principle (VO2 I Fick) in critically ill patients.

Methods: Fourteen patients (10 men and 4 women, mean age 39.4 +/- 5.4 years) were analyzed, 5 of them trauma victims and 9 sepsis victims. The following mean scores were obtained for these patients: APACHE II = 21.3+/-1.8, ISS = 24.8+/-6, and sepsis score = 19.6+/-2.3. The mortality risk (odds ratio), calculated from APACHE II, was 41.9+/-7.1%. All patients underwent mechanical ventilation and invasive hemodynamic monitoring with a Swan-Ganz catheter. VO2 was obtained using the 2 methods (VO2I Delta and VO2I Fick) at 4 different times (T1-T4).

Results: A good correlation was found between the 2 methods (r=0.77) for the mean of the 4 serial measurements. No statistically significant differences were observed between indirect calorimetry and Fick's equation at T1 (VO2I Delta = 138+/-28 and VO2I Fick = 59+/-38 mL.min-2.m-2, P=0.10) and T3 (VO2I Delta = 144+/-26 and VO2I Fick = 158+/-35 mL.min-2.m-2, P=0.14), but a significant difference was observed at T2 (VO2I Delta = 141+/-27 and VO2I Fick = 155+/-26 mL.min-2.m-2, P=0.03) and T4 (VO2I Delta = 145+/-24 and VO2I Fick = 162+/-26 mL.min-2.m-2, P=0.01).

Conclusion: We may state that indirect calorimetry can be used for oxygen consumption analysis in critically ill patients and is as efficient as Fick's reverse equation, with the benefit of being a noninvasive and risk-free procedure.